In a nutshell, network optimizations. For the most part, any actor with bReplicates set that is spawned on the server will replicate to relevant clients. BUT, within that actor, only properties that are flagged for replication with UPROPERTY(Replicated) are valid on clients. If every property was sent you would flood your network bandwidth very quickly. So UE4 actors need to opt-in their properties. If you haven’t already I strongly suggest reading this page: